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  #1  
Old 07-08-2012, 03:07 PM
capt55 capt55 is offline
 
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Flanking Rudders

Can someone answer this question for me?
Flanking rudders effect a vessel's heading because of the (one of the next answers)
1. effect of the propeller flow on the rudders.
2. water flo due to the vessel's movement through the water.
3.tunnel effect of the water flow past opposing rudders.
4.discharge current being channeled to im pinge on the vessel's deadwood.
Which answer fits best?

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  #2  
Old 07-08-2012, 05:06 PM
SamSam SamSam is offline
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I'm thinking primarily 1, secondarily 2. Number 3 I don't understand, number 4 sounds counterproductive.

There seems to be conflicting ideas on what they are and how they're used. I first heard the term 'flanking' to describe a maneuver on old sternwheel towboats. Originally they only had rudders in front of the paddle wheel like this...



When going forward their steering was only using #2, which at times was too weak for manueverability. In the case of pushing a line of barges around turns, (oddly enough, towboats pushed, they didn't tow barges) the flanking manuever developed, which as I understand it amounted to charging into the turn at speed and at the correct time throwing the paddle wheel into reverse and turning the steering wheel in the opposite direction normally required. That would allow the inertia/momentum of the tow to keep it going forward into the turn while the effects of #1 would kick the stern of the tow around until the whole thing was lined up better for the turn.

Eventually they developed 'Monkey Rudders' behind the paddle wheel like this...



which 'aped' the forward rudders and allowed the towboats the effects of #1 without losing forward speed. If you google flanking rudders those aft monkey rudders on sternwheelers are sometimes referred to as flanking rudders. In modern usage with regular propellers, the forward rudders are called the flanking rudders.

I've been told by one person that the two rudder systems turn in opposite directions, but to me that makes no sense at all and would counteract each other. I've seen in google images where for modern rc tugboat models, when going forward only the aft rudders work and when going astern only the forward/flanking rudders work. So they all obviously use #1 primarily, and it seems to me if the opposite rudders also turn in the same direction they would add some benefit from #2 instead of counteracting the other rudders.

I also saw where if the two modern flanking rudders on each side of the propeller were set to 'toe out' from each other a little bit, they tend to funnel water to the prop for a noticeable increase in thrust.
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  #3  
Old 07-09-2012, 12:09 PM
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johneck johneck is offline
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The flanking rudders help when the props are turning astern. The flow over the rudders is accelerated by the prop race. The normal rudders at that point are not in the race and thus much less effective. These are normally found on river towboats that operate at very low speeds and don't have alot of room for rudders, so they need the prop race to provide much turning force.
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Old 07-09-2012, 01:20 PM
SamSam SamSam is offline
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Do all the rudders operate all the time?
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Old 07-09-2012, 01:57 PM
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johneck johneck is offline
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They are actuated independently from the main rudders. As for exactly how and when they are used, I suspect that it varies greatly depending on the boats and the operators. If they are used when the boat is thrusting ahead, they will greatly disturb the inflow to the props causing cavitation and vibration due to the close proximity to the props.
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